The recent trends toward lighter, smaller, and higher-density electronic products have increased the demand for various printed wiring boards. In particular, the demand for a flexible printing wiring board (hereinafter, also referred to as “FPC)”) has shown a notable increase. The flexible printed wiring board is constituted from an insulating film and a circuit formed from a metal foil disposed on the film.
Typically, the flexible metal-clad laminate, from which the flexible printing wiring board is produced, is produced by bonding a metal foil onto a surface of a substrate with an adhesive material under heating and pressure, the substrate being a flexible insulating film made from an insulating material of various kinds. Polyimide films and the like are preferred as the flexible insulating film, and thermosetting adhesives such as epoxy and acrylic adhesives are typically used as the adhesive material. (Hereinafter, FPCs made using thermosetting adhesives are also referred to as “three-layered FPCs”.)
Thermosetting adhesives are advantageous in that bonding at relatively low temperatures is possible. However, requirements for properties, such as heat resistance, flexibility, and electrical reliability, are becoming more stringent, and it is possible that three-layer FPCs using thermosetting adhesives will have difficulty in meeting these stringent requirements. In order to overcome this problem, FPCs (hereinafter also referred to as “two-layer FPCs”) using thermoplastic polyimide as the bonding layer or made by directly forming a metal layer on the insulating film have been proposed. The two-layer FPCs have properties superior to those of the three-layer FPCs, and the demand for the two-layer FPCs is expected to grow in the future.
A flexible metal-clad laminate for a two-layered FPC may be prepared by a casting method, a metalizing method, or a laminating method, for example. In the casting method, a polyamic acid, which is a precursor of a polyimide, is cast and spread on a metal foil and is imidized. In the metalizing method, a metal layer is formed directly on a polyimide film by sputtering or plating. In the laminating method, a polyimide film and a metal foil are adhered together via a thermoplastic polyimide. The laminating method is more excellent than the others because it can provide a wider range of thickness of the metal foil than does the casting method and requires a lower apparatus cost than the metalizing method. As an apparatus for the lamination, heat roll laminating apparatus, double belt press apparatus or the like is used, in which materials in a rolled form are continuously unrolled to be fed in for the lamination. For better productivity, the heat roll laminating method is more preferable among them.
In the production of the conventional three-layered FPC by the laminating method, it is possible to carry out the lamination at laminating temperatures less than 200° C. (see Patent Document 1), because a thermosetting resin is used to form the adhesive layer. On the other hand, a thermoplastic polyimide is used to form the adhesive layer in the two-layered FPC. Therefore, it is necessary for the two-layered FPC to apply a high temperature of 200° C. or higher, in some cases, a temperature of approximately 400° C., in order to heat the adhesive layer to be adhesive. This induces distortion remained in the resultant flexible metal-clad laminate obtained by the lamination. Such distortion would result in dimensional changes when the flexible metal-clad laminate is wired by etching and when solder reflow is carried out for mounting elements on the resulting flexible printing wiring board.
Particularly in the laminating method, heat is continuously applied to imidize a polyamic acid, which is a precursor of a thermoplastic polyimide, in order to form an adhesive layer including the thermoplastic polyimide on the polyimide film, after the polyamic acid is cast and spread. Moreover, heat and pressure are continuously applied again, in order to laminate the metal foil. Therefore, heat is often applied to materials under tensile force. As a result, different heat stresses are produced in an MD direction and a TD direction. More specifically, the lamination applies a force on the film to expend it in the MD direction to which the tensile force is applied but to shrink it in the TD direction. As a result, this distortion is released when etching the metal foil away from the flexible metal-clad board to be wired and when heating the flexible printed wiring board by solder reflow. The release of the distortion causes shrinkage in the MD direction and expansion in the TD direction.
Recently, the wiring on the substrates is getting finer and finer in order to achieve reductions in size and weight of electronic devices. Accordingly, smaller elements are mounted in higher densities. Therefore, a large dimensional change after the formation of the fine wiring would shift the wires from positions to which the elements are to be mounted according to design, thereby failing to connect the elements with the substrate sufficiently.
In view of this, effort is made in order to suppress the dimensional changes by controlling a pressure to be applied in lamination with pressure or controlling the tensile force on the adhesive film (See Patent Document 2 or 3). Although the aforesaid means improves the dimensional changes, the improvement is not enough and further improvement is expected to suppress the dimensional changes.
Recently, in particular, a required level tends to become severe in solder resistance against moisture absorption, due to introduction of lead-free solder. Although, in order to clear the requirement, the adhesive layer is being changed so as to have higher Tg (glass transition temperature), the temperature necessary for the lamination becomes inevitably high consequently. Therefore, because the heat stress on the materials becomes larger, the materials are prone to the dimensional changes. Accordingly, it becomes necessary to design the material so that the heat stress is more efficiently suppressed.
At present, the insulating layer used for the two-layer FPCs has thickness of 25 μm (1 mill) in the main. However, as to the thickness of the insulating layer, there is uprising demand for reduction to equal to or less than 15 μm, in other words, a demand for “a half mill”, in order to solve problems such as further reduction of a mounting space on the substrate and spring-back or the like. Because the thickness of the adhesive film becomes thinner in the “half mill” two-layer FPCs, influence from the heat stress becomes larger at the lamination. Accordingly, it is more difficult to improve the dimensional changes of the “half mill” two-layer FPCs, compared with the one mill two-layer FPCs.
[Patent Document 1]
Japanese Unexamined Patent Publication, Tokukaihei, No. 9-199830
[Patent Document 2]
Japanese Unexamined Patent Publication, Tokukai, No. 2002-326308
[Patent Document 3]
Japanese Unexamined Patent Publication, Tokukai, No. 2002-326280.